Interactions of high energy photons with matter

1
Interactions of high energy photons with matter

• Cross section/attenution length/optical depth
• Photoelectric absorption
• Compton scattering
• Electron-positron pair production

2
Cross section
Cross-section per atom (or per electron)
? Attenuation length l 1/n? , where n is
density of atoms Attenuation of beam I I0
exp(-x/l) For materials, we often use the
attenuation coefficient, ?, which is the cross
second per mass (cm2/g) Then attenuation length l
1/n? 1/??, where ? is density Useful web
site for photon cross sections is
http//physics.nist.gov/PhysRefData/Xcom/Text/XCOM
.html
3
Three interactions

• Photoelectric absorption
• Photon is absorbed by atom
• Electron is excited or ejected
• Compton scattering
• Photon scatters off an electron
• Pair production
• Photon interacts in electric field of nucleus and
  produces an e e pair

4
Photoelectric cross section in Xe
5
Photoelectric absorption
Edges occur at the characteristic electronic
transition energies
When in emission, elements produce characteristic
lines at these energies
6
Photoelectric absorption
The photon electric cross-section scales with
Z5 This means that high-Z detectors are more
efficient at high energies. Above the highest
edge, the cross-section scales roughly as
(energy)-3. This means that photo-absorption
detectors rapidly become inefficient at high
energies.
7
Interstellar absorption
8
Compton scattering
9
Compton scattering
Inverse-Compton scattering, where the electron
transfers energy to the photon is an important
astrophysical process
10
Pair Production
Nucleus is needed for process to converse
momentum and energy
Only process with cross section which never
decreases with energy, dominates at high energies
11
Photon Cross Sections in Nitrogen
12
Photon Cross Sections in Lead
13
Reading

• Longair 4.1, 4.2, 4.3 (skip 4.3.3, 4.3.4), 4.4